Call transfer failign

Support Help
1

I’m experiencing a persistent issue with the transfer_call function. All transfer attempts fail immediately with a SIP 603 “Decline” error, and destination numbers are never actually called.

Error Message: error: Error transferring call, SIP status code: 603 SIP error category: user_declined Error: twirp error unknown: unexpected status from INVITE response: sip status: 603: Decline

What I’ve Tried:

  • Tested with two different destination numbers - both fail with same error

  • Tried both warm and cold transfer modes

  • Verified caller ID is set to “Retell Agent’s number”

  • Confirmed numbers are in E.164 format

  • Double-checked all transfer function settings

  • Using a Retell-purchased number (not custom telephony)

The agent immediately tells the caller the transfer failed, and the destination never rings. This happens consistently across all attempts.

Any help would be appreciated!

2

The 603 “Decline” error indicates that the transfer destination is actively rejecting the incoming call(1)(2). This is different from a busy signal or timeout - the destination is specifically declining to accept the transfer.

Since you’re using a Retell-purchased number with the agent’s number as caller ID, here are the most likely causes:

Telephony Provider Configuration

Even though you’re using a Retell-purchased number, the issue may be with how your telephony provider is configured. For cold transfers, Retell uses SIP REFER, and it’s up to the telephony provider to support caller ID override for SIP REFER(1)(2). Retell Twilio numbers support showing the user’s number on both warm and cold transfer, while Retell Telnyx numbers only support this when using SIP REFER via cold transfer(1)(2).

If caller ID override is not supported by the provider, the transfer will fail(1)(2).

Destination Number Issues

The destination numbers you’re transferring to may be:

  • Blocking calls from your Retell agent’s number
  • Using spam filtering that’s rejecting the calls
  • Not accepting calls from VoIP or cloud telephony sources

Recommended Troubleshooting Steps

  1. Test the destination numbers manually - Call them directly from your Retell agent’s number (outside of the transfer function) to verify they accept calls from that source

  2. Check transfer type settings - For warm transfers specifically, verify your configuration for on-hold music, human detection settings, auto-greet, and agent detection timeout(1)(2)

  3. Review SIP headers - You can add custom SIP headers for outbound calls, which are forwarded to your SIP provider on the SIP INVITE and can be used for custom routing(1)(2). All header names must start with X-(1)(2)

  4. Contact Retell Support - Since you’ve verified all standard settings and the issue persists, provide them with your call IDs and agent IDs for investigation

  5. Check telephony provider logs - Review logs in your telephony provider to see where the call routing is failing, and consider opening a ticket with them(3)

The fact that destination numbers are never actually ringing suggests the rejection is happening at the SIP/telephony layer before the call reaches the PSTN network.